1. Field of the Invention
The present invention relates to the plastic deformation of flat billets and more specifically to method and apparatus for equal channel angular extrusion (ECAE) of flat billets to control material structure, texture and physical-mechanical properties.
2. Description of the Prior Art
Historically, the primary goal of metalworking was to change billet shapes and dimensions. Thus, various forming operations such as forging, rolling, extrusion, etc. were developed. Simultaneously, plastic deformation has been recognized as an effective method for structure alteration and properties improvement of different metals and alloys. However, the conventional forming processes are not optimal to control material structures because of multiple reductions of a billet cross section, high pressures and loads, complicated machines and expensive tool which are necessary to attain high strains. The situation was dramatically changed after the applicant introduced materials processing by simple shear without any change of the billet cross-section. This technique known as equal channel angular extrusion (ECAE) comprises lateral billet extrusion between two intersecting channels of identical cross-sections. The method was first disclosed in the Invention Certificate of the USSR No 575892 of Oct. 22, 1974 and was described in other patents and publications (see, for example, Segal, U.S. Pat. No. 5,513,512, 1996; V. M. Segal et al “Plastic Working of Metals by Simple Shear”. English translation: “Russian Metallurgy”, No 1, pp. 99–105, 1981; V. M. Segal, “Materials Processing by Simple Shear”, “Mat.Sci&Eng.”, A 197, pp. 157–164, 1995). Multi-pass ECAE with special systems of billet orientation between passes allows one to accumulate severe strains and to optimize processing that provides unusual structural effects: grain refinement to sub-micron, sometimes to nano scale; refinement of second phases, hard particles, inclusions and precipitates; control of crystallographic textures and grains/phases aspect ratios; enhanced diffusivity and superplasticity; consolidation and bonding of powders; breakdown of cast structures; etc.
Originally, ECAE was introduced for long billets. Later, in the U.S. Pat. No. 5,850,755, 1998, the applicant also suggested ECAE of flat billets (FIG. 1). The last invention presents significant practical interest as it may be applied for fabrication of plates, sheets, strips and similar products. However, to make ECAE cost effective in high volume production and for massive billets, some problems of the technology should be resolved.
First, as that was described in the U.S. Pat. No. 5,850,755, the optimal ECAE of flat billets requires their rotation 90 degrees about a perpendicular axis to a billet flat surface after each pass. Therefore, punch impressions at this surface are mutually perpendicular (FIG. 2). These impressions reflect sharp changes of extrusion directions and grain orientations. That develops large local stresses near areas “A” of their intersections and may result in deep cracks during multi-pass processing.
Second, the prior art does not provide simple and effective means for billet ejection from the ECAE die. An extended contact area and high friction into the second channel requires a large ejection force to move the billet along the channel. That results in unreliable ejectors and complicated tool (see, for example, U.S. Pat. No. 5,85,755).
Third, contact friction into the first channel also presents a problem. The normal pressure on channel walls is proportional to friction coefficient and usually exceeds the material flow stress a few times even with the best lubricants. For large billets, the situation is more complicated because lubricant is carried away during a long stroke. In these cases, the ECAE die should be sufficiently strong, massive and expensive.
Forth, each pass of ECAE changes slightly a billet shape and may introduce flashes along a punch and movable die parts. Thus, some billet reshaping and debburing of flashes are necessary to insert the billet into the die and to eliminate laps, surface cracks and other defects. These operations are time and labor consuming, especially for large billets and warm/hot processing conditions.